1. Field of the Invention
The present invention relates to a method and apparatus for adjusting a zero point of a pressure sensor of an injection apparatus.
2. Description of the Related Art
Conventionally, an injection apparatus is disposed in an injection molding machine. A screw is disposed within a heating cylinder of the injection apparatus such that the screw can be rotated and reciprocated by drive means. A spiral flight is formed on the outer circumferential surface of the body of the screw; i.e., the screw body, so that a groove is formed by the flight.
When the screw is rotated in a forward direction during a metering step, pellets of resin which have fallen from a hopper attached to the heating cylinder are melted within the heating cylinder, and molten resin is caused to advance along the groove. As a result, the screw is retracted, and the molten resin is accumulated on the front side of a screw head. When the screw is advanced during an injection step, the resin accumulated in front of the screw head is injected from an injection nozzle and charged into a cavity of a mold unit. Incidentally, in the case in which the outer circumferential surface of the screw and the inner circumferential surface of the heating cylinder are of equal surface roughness, when the screw is rotated during the metering step, the resin within the groove rotates together with the screw and does not advance. Therefore, in general, the inner circumferential surface of the heating cylinder is made rougher than the outer circumferential surface of the screw.
When speed control or pressure control is performed during the injection step, pressure of resin is detected by use of a pressure sensor such as a load cell, and the detected pressure is used for feedback control.
Such pressure detection involves a problem as follows. In the course of manufacture of an injection apparatus, a load cell is built into the injection apparatus such that a value output from the load cell (i.e., a detection value) indicates zero. However, since mechanical resistance of the injection apparatus and frictional resistance associated with resin produce loads which act on the load cell, the actual resin pressure is not zero, resulting that the detection value indicates a pressure differing from the actual resin pressure. In order to solve this problem, an operation of adjusting the zero point of the load cell is performed.
FIG. 1 is a chart used for description of the principle of a conventional operation for adjusting the zero point of a load cell. In FIG. 1, the horizontal axis represents time, and the vertical axis represents detection value.
First, a screw is retracted at a predetermined speed from a predetermined position; e.g., a position at which a metering step has been completed (hereinafter referred to as a xe2x80x9cmetering completed positionxe2x80x9d), and a detection value PA output from the load cell during the retraction movement is detected. Subsequently, the screw is advanced at a speed equal to the predetermined speed, and a detection value PB output from the load cell during the advance movement is detected. A mean value POFF of the detection values PA and PB expressed by the following equation assumes a value which is not affected by mechanical resistance or frictional resistance associated with resin.
POFF=(PA+PB)/2
In an injection step, pressure of resin is detected by use of the load cell, while the mean value POFF regarded as a zero point, is subtracted from a detected pressure value, whereby actual resin pressure can be detected.
However, in the conventional injection apparatus, since the inner circumferential surface of the heating cylinder is made rougher than the outer circumferential surface of the screw, when the screw is advanced or retracted, a large frictional resistance acts on resin in the vicinity of the inner circumferential surface of the heating cylinder. Thus, as indicated by broken lines in FIG. 1, the detection values PA and PB change due to loads stemming from the frictional resistance and serving as disturbance. As a result, the mean value POFF involves variation, which renders performance of precise zero-point adjustment for the load cell impossible.
An object of the present invention is to solve the above-mentioned problem in the conventional injection apparatus, and to provide a method and apparatus for adjusting a zero point of a pressure sensor of an injection apparatus, which method and apparatus enable precise zero-point adjustment for the pressure sensor.
In order to achieve the above object, the present invention provides a method of adjusting a zero point of a pressure sensor of an injection apparatus, the method comprising the steps of: detecting pressure of resin by use of the pressure sensor, while moving a screw, in order to obtain a detection value corresponding to a detected pressure; and adjusting the zero point of the pressure sensor on the basis of the detection value.
The detection value is obtained when the screw is moved in a state in which a flight speed is rendered lower than a screw speed.
In this case, since the frictional resistance which acts on resin in the vicinity of the inner circumferential surface of the heating cylinder upon movement of the screw can be decreased, loads stemming from the frictional resistance are prevented from serving as disturbance against the detection value. As a result, zero-point adjustment for the pressure sensor can be performed precisely.
Preferably, the screw is advanced or retracted in a first stage, and is moved in the opposite direction in a second stage.
More preferably, the screw speed in the first stage is made equal to that in the second stage.
In this case, since the injection apparatus can be operated under the same conditions in the first and second stages, the zero-point adjustment for the pressure sensor can be performed precisely.